When a vehicle is driven with one of a pair of left and right driving wheels being on a low μ road surface, or when a vehicle is driven with one of a pair of left and right driving wheels not sticking to the road due to bumps on the road surface, the one of the pair of left and right driving wheels may slip. In such a case, the driving torque may not be applied to the non-slipping driving wheel because a differential device operates so as to apply a large driving torque to the slipping driving wheel and the wheel spins freely. Thus, when slippage occurs at one of a pair of left and right driving wheels, a slip restraining means restrains the slippage at the driving wheel.
A known slip control apparatus sets a target vehicle speed when a driver does not intend to accelerate, decelerate, or stop a vehicle. The known slip control apparatus adjusts the amount of the driving torque applied to driving wheels and the amount of the braking torque applied to individual wheels, thereby controlling the vehicle speed so as to correspond to the target vehicle speed. This control allows the vehicle to be driven at a constant speed when the driver does not intend to accelerate, decelerate, or stop the vehicle. Since the vehicle speed is calculated based on wheel speeds of the individual wheels, when slippage occurs at a driving wheel, the vehicle speed increases in conjunction with an increase in a rotation speed of the slipping driving wheel. Thus, when the slippage occurs at the driving wheel, the driving torque is controlled to be reduced so that the vehicle speed corresponds to the target vehicle speed. (For example, refer to JP 2004-90679A). Namely, in the slip control apparatus disclosed in JP 2004-90679A, the slip restraining means restrains the slippage by reducing the driving torque.
In another known slip control apparatus, when accelerating operation and braking operation are simultaneously conducted and slippage occurs at one of a pair of left and right wheels, braking torque is applied to a non-slipping driving wheel by operating the brake and also braking torque, which is larger than the braking torque generated by the braking operation, is applied to a slipping driving wheel (For example, JP H8-164834A). Thus, in the known slip control apparatus disclosed in JP H8-164834A, the slip restraining means restrains the wheel slippage by applying the braking torque to the slipping driving wheel.
Further, in another known slip control apparatus, when slippage occurs at a driving wheel, braking torque is applied to a slipping driving wheel and driving torque is reduced, (For example, refer to JP S61-85248A). Thus, in the known slip control apparatus disclosed in JP S61-85248A, the slip restraining means restrains the wheel slippage by applying the braking torque to the slipping driving wheel and reducing the driving torque.
In the slip control apparatus disclosed in JP 2004-90679A, the slip restraining means reduces the driving torque. Thus, when the slippage occurs at the one of the pair of left and right wheels, the driving torque of the non-slipping driving wheel is reduced due to the reduction of the driving torque to decrease the vehicle speed. Accordingly, the vehicle speed becomes lower than the target vehicle speed and hinders the vehicle from being driven at the demanded target vehicle speed.
In the slip control apparatus disclosed in JP H8-164834A, the slip restraining means applies the braking torque to the slipping driving wheel, and a state, in which the driving torque is hindered from being transmitted to the non-slipping driving wheel, is prevented by means of the differential device. However, the driving torque is consumed for applying the braking torque and hence the vehicle speed is lowered. Therefore, the driver is unable to drive the vehicle at a demanded vehicle speed by operating an accelerator.
In the slip control apparatus disclosed in JP S61-85248A, the slip restraining means reduces the driving torque and applies the braking torque to the slipping driving wheel. The reduction of the driving torque is conducted in the same mariner as that of the slip restraining means disclosed in JP 2004-90679A, and the braking torque is applied to the slipping driving wheel in the same manner as that of the slip restraining means disclosed in J-P H8-164834A. Thus, in the slip control apparatus disclosed in JP S61-85248A, as in TP 2004-90679A and JP H8-164834A, when the slippage occurs at the one of the pair of left and right wheels, the vehicle speed lowers and the driver is unable to drive the vehicle at the demanded vehicle speed by operating the accelerator.
The present invention has been made in view of the above circumstances, and provides a slip control apparatus and a slip control method, which enable a vehicle to be driven at a demanded vehicle speed by restraining vehicle speed reduction when slippage occurs at driving wheel(s).